1. Field of the Invention
This invention relates to a process for producing a corrosion resistant colored surface on an aluminum or aluminum alloy substrate.
2. Description of the Prior Art
Anodized aluminum, formed by the electrolytic treatment of aluminum or aluminum alloys in a sulfuric acid bath may be treated by a secondry electrolytic treatment in a coloring bath containing metallic salts, as is described in U.S. Pat. No. 3,382,160. It is believed that this process results in the precipitation of the metal salts in the pores of the anodic coating on the aluminum substrate. Aluminum treated by this process produces colored materials useful for construction and other applications, but the color series which may be obtained is limited to a bronze series merging into black.
The codeposition of nickel and zinc from a plating bath containing nickel sulfate and zinc sulfate, at a pH of 2-4, onto has been reported by K. Mitsuo et al, Electrochemistry 45, No. 12 (1977), pp. 728-733 and Electrochemistry 47, No. 2 (1979) pp. 89-94. According to these papers, the plated coating of an intermetallic compound of nickel and zinc are formed on a rolled thin copper plate. The color of the plated coating disclosed therein has a silver-white tint and it has not been found possible heretofore, using such a plating process, to obtain a pure grey series coating.
There remains a long-felt need to prepare secondarily colored anodized aluminum products of a true grey series, without bronze tint and with enhanced corrosion resistance.